The purpose of this study was to discover which of nine diets would produce the largest mean brood size in Daphnia magna. Two media, one inorganic and one organic, were employed to culture three green algae: Selenastrum capricornutum, Ankistrodesmus convolutus, and Chlamydomonas reinhardtii. In addition to single algal foods, mixtures of algae were also employed. These consisted of S. capricornutum and C. reinhardtii, which were fed in varying quantities. A synthetic diet of fish chow and alfalfa served as a control. All algal diets were bacteria-free.

Neonates were acclimatized in autoclaved Lake Huron water and reared on one of the nine diets for five weeks. Demographic studies were carried out on the progeny of these acclimatized adults. First broods began to arrive after seven days. Thereafter, every other day, neonates were collected, counted, and weighed. These data presented in this study are based on 49 982 neonates.

Daphnia magna consuming algae reared on inorganic medium were less productive than those consuming algae reared on organic medium. The largest mean brood size was obtained from the mixed algal diet reared on the organic medium while the smallest mean brood size was produced by the synthetic diet. Large broods generally consisted of lighter individuals (10.8 μg per neonate) than those produced in smaller broods (20 μg per neonate). The evolutionary significance of this observation, as well as its effect on chronic life cycle studies, is discussed.